Optical Diagnostics and Imaging - Applied Optics Fundamentals and Device Applications - page 315

6.13.2 Flash X-Ray
Flash x-ray is a well-established technology in which an electron beam diode
(anode and cathode) in a sealed tube produces a brief (<30 ns) intense x-ray
pulse. Recovery times are too slow to repetitively pulse the anode for high-
speed cineradiography—high frame speed requires multiple independent
pulsers and diodes. Sources are closely packed to minimize parallax problems
and to maintain source intensity with smaller diodes [15]. Triggering of x-ray
pulses is tailored to the event timing, so that precise imaging of the event is
achievable with either high-speed framing cameras in which image frames are
matched to the x-ray pulses, or high-speed video cameras with suitable expo-
sure rates. Image processing algorithms are then utilized to allow extraction
of parametric data from multiple frame x-ray images produced on scintillator
screens and images through microchannel plate or image intensifier hybrids.
X-ray computed tomography (XCT) represents an alternative approach
with the advantage of a 3-D image. The technique requires four or more
pulsers for each 3-D frame, translating to 32 or more pulsers, to produce
an equivalent eight frames [16]. There is no commercially available anode
arrangement that lends itself to the correct geometry, and the cost of such
a system would be substantial. For this reason, custom systems are con-
structed for specific applications.
6.13.3 Typical System Requirements
System specifications for cineradiography systems in development include
the following:
• Multi-anode x-ray system, 150-450 keV x-ray photons
• Fluorescent screen/real-time video camera imaging, 6-8 foot
standoff
• One to five mm spot size, with 1 m 2 target area
• Up to 4 ms record time
• Eight images at 100,000 frames per second, trigger synch pulses
• Pulser repetition rate adjustable from 10,000 to 100,000 s −1
• Independent inter-pulse time from anode to anode
• Adaptable to a wide range of test scenarios/operation in harsh con-
ditions/explosive test environment
Shown in Figure 6.20 is a multi-anode arrangement where the acquisi-
tion is achieved with a fast scintillator and a high-speed video camera.
Multiple frames of imagery are produced on a scintillator screen by flash
x-ray exposure through the test materials at either 150 or 450 keV. A turn-
ing mirror is used to avoid placement of the video camera directly in the
residual x-ray beam. In a former approach, separate x-ray heads were